Ion exchange active fluoropolymer films have been widely used in industry, particularly as ion exchange membranes in chlor-alkali cells. Such membranes are made from fluorinated polymers having sites convertible to ion exchange active groups attached to pendant groups from the polymeric backbone.
Such polymers are usually thermoplastic and may be fabricated into films or sheets while in their molten form using mechanical extrusion equipment. However, such equipment is operated in the temperature region near the crystalline melting point of the polymer, which is commonly near the decomposition temperature of some of the polymers. Thus, decomposition may be a problem when some polymers are formed into films by conventional methods. Likewise, it is difficult to make such polymers into films thinner than about 10 microns using such techniques. In addition, it is difficult to make films of consistent thickness. It would be highly desirable to be able to make thin films having a consistent thickness.
Forming membrane structures and support structures into multiple layers is the subject of several patents and applications including U.S. Pat. Nos. 3,925,135; 3,909,378; 3,770,567; and 4,341,605. However, these methods use complicated procedures and equipment including such things as vacuum manifolds, rolls and release media.
Prior art methods for fabricating films from perfluorinated polymers have been limited by the solubility of the polymers and the temperature-dependent viscosity-shear rate behavior of the polymers. To overcome these characteristics of perfluorinated carboxylic ester polymers, workers have tried to swell the polymers using various types of swelling agents and to reduce the fabrication temperatures of the polymers to practical ranges by extraction. Extraction methods have been taught in, for example, U.S. Pat. No. 4,360,601. There, low molecular weight oligomers were removed from carboxylic ester polymers. Polymer "fluff" was extracted in a Soxhlet device at atmospheric pressure for 24 hours (see Examples 1 and 3 of U.S. Pat. No. 4,360,601). Such treatments has been found to make some fluorinated carboxylic ester copolymers more processible and operate more efficiently in a chlor-alkali cell when in a hydrolyzed form. Such extractions modify the fabricated polymer article, for example, by forming grease of the polymer as shown in Example 3 of U.S. Pat. No. 4,360,601.
In addition, such extractions seem to lower processing temperatures of carboxylic ester polymers after isolation. Isolation means separation from the polymerization latex by conventional methods of deactivating the surfactant such as freezing, heating, shearing, salting out or pH adjustment.
British Patent 1,286,859 teaches that highly polar organic "solvents" dissolve small amounts a fluorinated vinyl ether/tetrafluoroethylene copolymer in its thermoplastic form. Thermoplastic form means the polymer is in a form which can be molded or processed above some transition temperature (such as the glass transition temperature or the melting point) without altering its chemical structure or composition. The patent teaches the use of the following materials "solvents": butanol, ethanol, N,N-dimethylacetamide, and N,N-dimethylaniline.
Similar approaches have been used to swell membranes in their ionic forms. Ionic forms of membranes are membranes which have been converted from their thermoplastic form (--SO.sub.3 F or --COOCH.sub.3) to their ionic forms (--SO.sub.3 M or --COOM) where M is H.sup.+, K.sup.+, Na.sup.+, or NH.sub.4 .sup.+ or other metal ion.
Prior art workers have used highly polar solvents or mixtures of solvents on substantially perfluorinated polymers and less polar solvents on fluorinated polymers containing hydrocarbon components as co-monomers, ter-monomers or crosslinking agents.
However, each of the prior art methods for swelling, dispersing or extracting the polymers has certain shortcomings which are known to those practicing the art. Polar solvents have the potential for water absorption or reactivity with the functional groups during subsequent fabrication operations, thus making poor coatings, films, etc. High boiling solvents are difficult to remove and frequently exhibit toxic or flammability properties. Functional form (ionic forms) of the polymers can react with solvents. (See Analytical Chem., 1982, Volume 54, pages 1639-1641).
The more polar of the solvents such as methanol, butanol esters, and ketones as used in U.S. 3,740,369; British Patent 1,286,859; and Chemical Abstracts 90:169702 have high vapor pressures at ambient conditions, which are desirable for solvent removal; however, they tend to absorb water. Their water content is undesirable because it causes problems in producing continuous coatings and films of hydrophobic polymers. In addition, polar solvents frequently leave residues which are incompatible with the polymers. Also, they frequently leave residues which are reactive during subsequent chemical or thermal operations if they are not subsequently removed.
Another approach taken by the prior art workers to form films from fluoropolymers include the use of high molecular weight "solvents" which have been produced by halogenating vinyl ether monomers. (See British Patent 2,066,824A).
The swelling of the functional (ionic) forms of the fluoropolymers by polar or hydrophilic agents has been known for some time. In addition, the solvent solubility parameters were compared to the swelling effect of 1200 equivalent weight Nafion ion exchange membrane (available from E. I. DuPont Company) by Yeo at Brookhaven Laboratory (see Polymer, 1980, Volume 21, page 432).
The swelling was found to be proportional to two different ranges of the solubility parameter and a calculation was developed for optimizing ratios of solvent mixtures. Ionic forms of functional fluoropolymers may be treated in such a manner, however, the subsequent physical forming or manipulation of the polymers into usable configurations by any thermal operation is limited when the polymers are in the functional forms. In addition, non-ionic forms of polymers treated in this manner are also limited in the thermoplastic processing range by the stability of the functional group bonds.
Other solvation methods have used temperatures near the crystalline melting points of the polymers being solvated, thus requiring either high boiling point "solvents" or high pressure vessels to maintain the system in a solid/liquid state. See Analytical Chem., 1982, Volume 54, pages 1639-1641.
Burrell states the theory of Bagley [J. Paint Tech., Volume 41, page 495 (1969)]predicts a non-crystalline polymer will dissolve in a solvent of similar solubility parameter without chemical similarity, association, or any intermolecular force. However, he fails to mention anything about the solubility of polymers demonstrating crystallinity.